Broadband microwave phase shifter utilizing stripline coupler

ABSTRACT

A TWO-PORT MICROWAVE-SIGNAL PHASE SHIFTER UTILIZES STRIPLINE TECHNOLOGY. THE PHASE SHIFTER COMPRISES A FIRST SIGNAL CONDUCTOR PRINTED IN THE FORM OF A CLOSED LOOP ON ONE SIDE OF A DIELETRIC SHEET. PRINTED ON THE OTHER SIDE OF THE SHEET IS A SECOND SIGNAL CONDUCTOR OVERLYING THE FIRST SIGNAL CONDUCTOR AND SIMILARLY CONTOURED. HOWEVER, THE SECOND SIGNAL CONDUCTOR IS AN OPEN LOOP. THE ENDS OF THE SECOND SIGNAL CONDUCTOR ARE THE SIGNAL TERMINALS. DISPOSED AGAINST EACH OF THE SIGNAL CONDUCTORS IS A SHEET OF DIELECTRIC MATERIAL HAVING A CONDUCTIVE LAYER ON ITS FACE REMOTE FROM THE SIGNAL CONDUCTOR.

C. W.*GERST Feb. 9, 1971 BROADBAND MICROWAVE PHASE SHIFTER UTILIZINGSTRIPLINE COUPLER Filed June 17, 1968 2 Sheets-Sheet 1 PRIOR ART 4 icoUPLERgg FIG.3

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PHASE SHIFTER 130 INVENTOR. Carl W. Germ` ATTORNEY C. W. GERST Feb. 9,1971 BROADBAND MICROWAVE PHASE SHIFTER UTILIZING STRIPLINE COUPLER FiledJune 17, 1968 2 Sheets-Sheet 2 Flag FIG.1 I

nited States Patent BROADBAND MICROWAVE PHASE SHIFTER UTILIZINGSTRIPLlNlE COUPLER Carl W. Gex-st, Skaneateles, NX., assignor to AnarenMicrowave Incorporated, Syracuse, NX., a corporation of New York FiledJune 17, 1968, Ser. No. 737,745

Int. Cl. HOIp 1/18, 5/14 U.S. Cl. S33-10 1l Claims ABSTRACT F THEDISCLOSURE This invention pertains to microwave-signal phase Shiftersand more particularly to microwave-signal phase Shifters ideally suitedfor stripline technology.

Microwave signal processing relies heavily on the use of phase shiftersand hybrids. In fact, the bulk of the signal processing is done with 90hybrids and 180 hybrids. Usually a 90 phase shifter is used to convert a90 hybrid to a 180 hybrid. Therefore, in reality, most microwave signalprocessing apparatus can be performed by 90 hybrids and 90phase-Shifters. However, in order to get broadband operation, certain 90hybrids and certain 90 phase-Shifters are desirable. With respect to thehybrids, the most desirable is a backward-wave transmission-line couplerbecause of its broad band characteristics. This device has various namesin the art, i.e. 3 db coupler, quadrature coupler, coupled transmissionline coupler, etc. A 90 phase-shifter with the desired dispersivecharacteristics is a Schiffmfan phase-1 shifter. In fact, the Schiffmanphase-shifter is obtained from modifying a backward-wavetransmission-line coupler.

While it is true that the backward-wave transmissionline coupler and theSchiffman phase-shifter readily lend themselves to stripline technology,a problem arises with the Schiffman phase-lifter when photo-etched orprinted circuit techniques are used to realize the stripline technology.In stripline technology two signal conductors are disposed in separateparallel planes between two parallel ground planes. Now the Schiffmanphase-shifter requires that the signal conductors overlie each other fora given coupled path distance, usually one-quarter of the centerfrequency wavelength and that they be physically short circuitedtogether at one point. In order to maintain the bandwidth and minimizereflections the short circuiting point is extremely critical. Thedistance which the actual point deviates from the desired point shouldbe an as small as possible fraction of the coupled path distance. Itshould be apparent that the higher the operating frequency (the shorterthe operating wavelength), the more likelihood of error. In fact, formillimeter wavelenths extreme precision is required.

When printed circuit techniques are employed the signal conductors areprinted or photo-etched on opposite sides of a sheet or dielectricmaterial. While the printing operation results in siganl conductorshaving the desired precision for the geometry there is a serious problemconcerning the short circuiting. In order that one 3,562,674 PatentedFeb. 9, 197i ice of the signal conductors be connected to the other, acut must be made through the sheet of dielectric material at the desiredpoint and a metallic tab inserted and soldered to both signalconductors. This operation is costly and is subject to error since itrequires a worker to precisely make the cut at the required point with ablade of finite thickness and often with the aid of a microscope.

It is accordingly a general object of the invention to provide animproved microwave-signal phase-shifter.

It is another object of the invention to provide an improvedmicrowave-signal phase-shifter which accomplishes the same end result asa Schiffman phase-shifter but which does not require that the two signalconductors be physically short circuited.

It is another object of the invention to replace the Schiffman phaseshifter by a stripline phase shifter employing printed circuit orphoto-etched techniques wherein no mechanical cut need be made in thedielectric sheet between the signal conductors so that the constructionof the phase shifter is only a printing operation.

It is a further object of the invention to satisfy the cited objectswith a broadband microwave-signal phase shifter which is extremelysimple, reliable and inexpensive to fabricate.

Briefly, the invention contemplates a two-part microwave signalphase-shifter utilizing a backward-wave transmissionline coupler. Thecoupler includes at least one ground plane element and rst and secondnon-intersecting conductors which are substatially parallel to theground plane element. The ground plane element and the signal conductorsare in electromagnetically cooperative relationship so that at least aportion of the microwave energy being guided by one signal conductor iscoupled to the other. Each end of each signal conductor is a signaltransfer port. One of the signal transfer ports of the first signalconductor is short circuited to the other signal transfer port of thefirst signal conductor, while one signal transfer port of the secondsignal conductor is adapted to receive microwave signals and the othersignal transfer port of the second signal conductor is adapted totransmit the received microwave signals.

It should be noted that such a phase-shifter is usable in striplinetechnology by utilizing two ground plane elements and by disposing thesignal conductors in different planes or in microstripline technology byutilizing one ground plane element and by disposing the signalconductors in the same plane.

Alternately, the microwave-signal phase-shifter can be viewed ascomprising a first signal conductor in the form of a closed loop whichis disposed in a rst plane and a second signal conductor disposed in asecond plane parallel to and displaced from the second plane. The secondsignal conductor has a signal transfer port at each of its separatedends. In addition, the second signal conductor at least partiallyoverlies the first signal conductor and has a similar path contour sothat the signal conductors are electromagnetically coupled. Both signalconductors are displosed between a pair of parallel ground planeelements to provide a stripline configuration.

It should be noted while the invention is ideally suited to printedcircuits or photo-etched realizations, it is also applicable toconventional microwave plumbing realizations.

Other objects, features and advantages of the invention will be apparentfrom the following detailed description of the invention when read withthe accompanying drawings which show, by way of example, and notlimitation the now preferred embodiment of the invention.

FIG. l shows a top view of a prior art backward-wave transmission-linecoupler using stripline technology;

FIG. 2 is a sectional View taken along the line 2-2 in FIG. l;

FIG. 3 is a sectional view taken along the line 3 3 of FIG. l;

FIG. 4 is a sectional view taken along the line 4 4 of FIG. l;

FIG. 5 is a top View of a prior art Schiffman phase shifter usingstripline technology;

FIG. 6 is a sectional view taken along the line 6 6 of FIG. 5;

FIG. 7 is a sectional view taken along the line 7 7 of FIG. 5;

FIG. 8 is a top view of a phase shifter in accordance with the inventionutilizing stripline technology;

FIG. 9 is a sectional view taken along the line 9 9 of FIG. 8;

FIG. 10 is a sectional view taken along the line 1(9 10 of FIG. 8;

FIG. ll is a top view of the phase shifter of FIG. 8 with the top groundplane element removed; and

FIG. l2 is a bottom view of the phase shifter of FIG. 8 with the bottomground plane element removed.

By way of background a stripline embodiment of a backwardwavetransmission-line coupler is shown in FIGS. l to 4 as coupler 30. Thecoupler 30 is linear and reciprocal. The coupler also has a givenbandpass and has a characteristic impedance at its signal transferterminals. Normally, the microwave-signal energy being processed hasfrequencies within the passband of the coupler and devices connected tothe coupler have input and output impedances which match thecharacteristic impedance of the couplers. For the sake of deniteness theterminals 31 and 32 are considered to be the input terminals of thecoupler and the terminals 33 and 34 are considered to be the outputterminals of the coupler. Because of the reciprocal nature of thecoupler, the input terminals and output terminals can be interchanged.

If a microwave signal is received at the rst input terminal 31 the poweror energy of the signal is split into two equal quantities. One quantityis fed to the rst output terminal 33 and the other is fed to the secondoutput terminal 34. The signal phase of the power transmitted fromoutput terminal 33 is advanced by 9() electrical degrees from the signalphase of the power transmitted from output terminal 34. Thus, if themicrowave power received at input terminal 31 is represented by thequantity A, the terminals 33 and 34 transmit microwave energy havingvoltages represented by the quantities A/\/2 and ]'A/\/2 respectively.Similarly, if a microwave signal is received at thev second inputterminal 32, the power of the signal is split into two equal quantities,one half of the power is fed to each of the output terminals 33 and 34.The signal phase of the power transmitted from output terminal 34 isadvanced by 90 electrical degrees from the signal phase of the powertransmitted from output terminal 33. Thus, if the microwave powerreceived at input terminal 32 is represented by the quantity B, theterminals 33 and 34 transmit wave power having voltages represented bythe quantities iB/V2 and B/\/ 2, respectively. If microwave-signal poweris simultaneously applied to input terminals 31 and 32, signalsuper-position occurs because the coupler is linear. Therefore, by usingthe above indicated terminology, when microwave power received at inputterminal 31 is represented by A and the microwave power received atinput terminal 32 is represented by B, output terminal 33 transmitsmicrowave power having a voltage represented by (-J'B) V2 and outputterminal 34 transmits microwave power having a voltage represented by 4Hence, the names 3 db coupler or 90 degree hybrid. Two points are worthrepeating: (l) any power received at an input terminal is dividedequally between the output terminals, and (2) the signals transmitted bythe output terminals have a degree difference.

Coupler 30 comprises a central sheet of dielectric material 50. On thetop surface of the sheet 50 is a first signal conductor 52 having threecontinguous portions 52A, 52B and 52C angularly disposed with respect toeach other. Conductor 52 is indicated by dot-dash lines in FIG. l. Onthe bottom surface of sheet 50 is a second signal conductor '54 havingthree contiguous portions 54A, 54B and 54C, angularly disposed withrespect to each other. Conductor 54 is indicated by dash lines in FIG.l, Portions 52B and 54B are in parallel opposed relationship. T heenergy transfer between the two conductors 52 and 54 occurs only viathese portions. The lengths of these portions are odd-integral multiplesof quarter operation wavelengths. The angular disposition of the otherportions is to prevent coupling at other regions. (It should be notedthat the angles are exaggerated.) It should be noted that the activeregion or the coupler per se is actually the portions 52B and 54B, theremaining portions are primarily signal leads. This fact is importantfor the subsequent discussions of the phase Shifters. The ends of theportions 52B and 54B are signal transfer ports which are connected tothe input and output terminals. In particular, one end of portion 52B isconnected via portion 52A to terminal 31; the other end of portion 52Bis connected via portion 52C to terminal 34. Similarly, one end ofportion 54B is connected via portion 54A to terminal 32; the other endof portion 54B is connected via portion 54C to terminal 33. Disposed ontop of conductor 52 is a sheet of dielectric material 56. On the top ofsheet 56 is a ground-plane element 58 in the form of a layer ofconductive material. Disposed below conductor 54 is a sheet ofdielectric material 60. Below sheet 60 is a ground-plane element 62 alsoin the form of a layer of conductive material.

Conductor 52 electromagnetically cooperates with groundplane elements 58and 62 to provide a transmission line of the shielded-stripling type;conductor 54 electromagnetically cooperates with ground-plane elements58 and 62 to provide a transmission line of the shielded-stripline type.Input terminal 31 is connected to one end of conductor 52; and inputterminal 32 is connected to one end of conductor 54. The outputterminals 33 and 34 are coupled to the other ends of conductors 54 and52 respectively.

The coupler 30 can be fabricated by photo-etching the conductors 52 and54 on opposite sides of a dielectric substrate having surfaces of aconductive material using conventional printed-circuit techniques andsandwiching this substrate between two other substrates havingconductive material on their outer surfaces. With such a coupler at 2:1bandwidth is easily obtained and with moderate care an 8:1 bandwidth canbe achieved.

The Schiffman phase sifter 130 shown in FIGS. 5, 6 and 7 is readilyderived from the coupler 30 of FIGS. 1 to 4. In fact, it is onlynecessary to apply feedback between terminals `31 and 33. Actually, thefeedback path shown can be minimized. Since the active region of thecoupler 30 is that associated with the opposed portions 52B and 54B, thefeedback is best applied by connecting together the ends of portions"52B and 54B which are closest to terminals 31 and 33. When this isdone, terminals 31 and 33 and portions 52A and 54C of signal conductors52 and 54 can be eliminated. With these facts in mind phase-shifter 130will now be described. However, because many of the elements ofphase-shifter 130 are similar to the elements of coupler 30, referencecharacters incremented by will be used for such similar elements andonly the difference will be described.

In particular signal conductor 154 comprises only elements 154A and 154Band signal conuctor 152 comprises only elements 152B and 152C. The endof the portion 154B of signal conductor 154 which is remote fromterminal 132 is connected by short-circuiting link 153 to the end of theportion 152B of signal conductor 152 which is remote from terminal 134.These ends are signal transfer ports. It is because of the need forshortcircuiting link 153 which must traverse the central sheet 150 thatfabrication problems arise when the phase shifter 130 is realized byusing photo-etching or printed circuit techniques. Again, it should berealized that the other ends of the portions 154B and 152B are alsosignal transfer ports which are connected via portions 154A and 152C toterminals 132 and 134 respectively.

Applicant has discovered that substantially the same desirabledispersive characteristics which result in a broad passband response canbe obtained if feedback is applied between signal terminals y31 and 34of coupler 30. Again it should be recognized that the feedback pathshould be short and that the active portions of the coupler isassociated with portions 52B and 54B of the signal conductors 52 and 54,with the remaining portions only serving as signal conduits.

With these facts in mind the phase shifter 230 according to theinvention is shown in FIGS. 8 to l2. Again elements similar to those ofcoupler and phase shifter 130 bear reference characters prefixed by thenumber 2.

In particular, coupler 230 compirses a sheet of dielectric material 250.On the top face of sheet 250, as viewed in the drawings, is printed orphoto-etched signal conuctor 254 comprising portions 254A, 254B and 254C(see FIG. ll). It should be noted that there is a gap between the endsof portion 254B. These ends are signal transfer ports. On the other facesheet 250 is printed or photo-etched a second signal conductorcomprising portions 252B and 253 connected to form a closed loop. Whilethe geometry shown is a circle, other closed-loop geometries can equallywell be employed, subject to the following criteria: portions 252B and254B should have relatively similar contours and lengths as required bythe need for electromagnetic coupling therebetween. Now, the junction ofportions 254A and 254B can be considered as a signal transfer port withportion 254A and terminal 232 comprising a signal path for access to thesignal transfer port; similarly, for the junction of portions 254B and254C. In addition, one junction of portion 252B and portion 253 can beconsidered as a signal transfer port, while the other junction can alsobe considered as a signal transfer port. In such a case the portion 253is a short circuiting element between the signal transport portsassociated with the coupled portion 252B. It should be noted thatportion 253 lies below the gap between the ends of portion 254B. Theamount of electromagnetic coupling is controlled by the thickness ofsheet 250, (the thinner the sheet the tighter the coupling) by theamount of overlaying congruence between the portions 252B and 254B(off-setting the overlie decreases the tightness of the coupling), andby the length of the gap betweenv the ends of portion 254B (andconsequently the length of portion 253).

Just as with coupler 30 and phase-shifter 130, the remainder ofphase-shifter 230 comprises two other sheets 256 and 260. Each sheet isof dielectric material. Each of the sheets is fixed against one of thefaces of sheet 250. The outer faces of each of sheets 256 and 260 arecovered with a layer of conductive material 258 and 262, respectively,to provide ground plane elements.

In order to enhance the coupling between the portions 252B and 254B ofthe signal conductors, the thickness and dielectric constant of thesheet 250 is less than the thickness and dielectric constant of thesheets 256 and 260. In addition, in order to provide impedance matchesbetween the various portions of the signal conductors theircharacteristic impedances are controlled by varying the width of thevarious portions. It should be noted that the uncoupled portions 254A,254C and 253 have substantially the same width which is approximatelytwice the Width of the coupled portions 252B and 254B.

In operation, microwave signals can be applied between terminal 232 andground plane element 258 and transmitted after a phase shift fromterminal 233 and ground plane element 258, and vice versa.

There has thus been shown an improved phase-shifter having a dispersivecharacteristic lwhich is substantially identical with the Schiffmanphase-shifter and consequently nearly constant phase shift over a broadband. However, the phase shifter of the invention has no crossoversbetween different planes and therefore is ideally suited to striplinetechniques requiring only printed circuit or photo-etching operationsand no mechanical intervention by a worker during fabrication.

While only one embodiment of the invention has been shown and describedin detail, there will now be obvious to those skilled in the art manymodifications and variations, satisfying many or all of the objects ofthe invention which do not depart from the spirit thereof.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A microwave-signal phase shifter comprising a first signal conductorin the form of a closed loop and disposed in a first plane, a secondsignal conductor having a terminal at each of the separated ends thereofand disposed in a second plane parallel to and displaced from said firstplane, the second signal conductor at least partially overlying saidfirst signal conductor and similarly contoured, said first and secondsignal conductors being electromagnetically coupled, a iirstground-plane element disposed in a third plane parallel to and displacedfrom said first plane on the side thereof remote from said second plane,a second ground-plane element disposed in a fourth plane parallel to anddisplaced from said second plane on the side thereof remote from saidfirst plane.

2. The microwave-signal phase-shifter of claim 1 wherein said secondsignal conductor has a given width, the portion of said first signalconductor opposite said second signal conductor has said given width,and the remaining portion of said first signal conductor hassubstantially twice said given width.

3. The microwave-signal phase-shifter of claim 1 wherein the spacingbetween said first and second planes is less than the spacing betweensaid first and third planes.

4. The microwave-signal phase-shifter of claim 3 wherein the spacingbetween said second and fourth planes is equal to the spacing betweensaid first and third planes.

5. The microwave-signal phase-shifter of claim 1 further comprising asheet of dielectric material disposed between said first and secondsignal conductors.

6. The microwave-signal phase-shifter of claim 5 wherein said first andsecond signal conductors are printed on said sheet of dielectricmaterial. 'd

7. The microwave-signal phase-shifter of claim 1 further comprising afirst sheet of dielectric material disposed between said first signalconductor and said first ground-plane element, and a second sheet ofdielectric material disposed between said second signal conductor andsaid second ground-plane element.

8. The microwave-signal phase-shifter of claim 7 further comprising athird sheet of dielectric material disposed between said iirst andsecond signal conductors.

9. The microwave-signal phase-shifter of claim 8 wherein the dielectricconstant of the dielectric material of said third sheet is less than thedielectric constant of the dielectric material of said first and secondsheets.

10. The microwave-signal phase-shifter of claim 9 wherein the dielectricconstant of the dielectric material of said second and third sheets isequal.

References Cited UNITED STATES PATENTS 1/1960 Cohn 333-10 8/1949 Tiley333--10X 8 8/1958 Kock 333-1OX 2/ 1959 Sferrazza S33-10 HERMAN KARLSAALBACH, Primary Examiner 5 P. L. GENSLER, Assistant Examiner U.S. C1.X.R.

